John H. Spang1,
Steven L. Dorobek2
(1) Texas A&M University, College Station, TX
(2) Texas A&M University
Abstract: Using Antithetic Normal Faults to Map Growth Axial Surfaces in Growth-faulted Sediments
Changes in the curvature of growth axial surfaces can reflect changes in sedimentation rate, fault displacement rate, or differential compaction of hanging wall growth strata. Increasing dip of the growth axial surface can reflect increasing sedimentation rates in the hanging wall depocenter, decreasing displacement rates along the growth fault or change to less compactible sediment on top of the hanging wall. The shape of rollover anticlines in the hanging wall reflects the curvature of the bend in the normal fault where the fold develops. A gentle bend results in a rounded anticline, whereas an angular bend forms an angular fold. Differential compaction of growth sediments over the anticline can dampen the amplitude and broaden the shape of the fold while increasing closure area. Unless the fold is very angular, the actual location of the growth axial surface is difficult to determine using only the geometry of folded layers. Antithetic faults that form at the fault bend commonly cut through pre-growth sediments, and in ideal examples, antithetic faults propagate to the top of growth sediments. The point where the antithetic fault intersects the top of the growth sediments defines the location of the growth axial surface at that instant. With continued displacement and sedimentation, additional antithetic faults form as older faults move away from the bend and are covered by younger, unfaulted growth sediments. The upper termination of each antithetic fault, however, reflects the position of the growth axial surface when that antithetic fault formed. When combined with high-resolution chronostratigraphy, this method represents a powerful tool for interpreting the history of listric normal faults.
AAPG Search and Discovery Article #90914©2000 AAPG Annual Convention, New Orleans, Louisiana